Post-operatively adjustable spinal fixation devices

ABSTRACT

A system for spinal fixation with a non-rigid portion at least one of the caudal or cephalad terminus. Various devices and techniques are described for transition from a rigid fixation construct to a less rigid support structure applied to a “soft zone” that helps share the stress created on the spinal levels caused by the fixed levels below. In some embodiments, the soft zone is provided by terminating the construct with one of a flexible tether or a dampening rod.

CROSS-REFERENCES

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/432,647 filed Feb. 14, 2017, which is a continuation ofInternational Application PCT/US17/17700, filed 13 Feb. 2017 that claimspriority to U.S. Patent Application No. 62/294,975, filed on 12 Feb.2016, the contents of which are entirely incorporated herein byreference.

BACKGROUND Field

The present disclosure relates generally to medical devices, andspecifically to surgical instruments and methods for performing spinalprocedures.

The spine is critical in human physiology for mobility, support, andbalance. The spine protects the nerves of the spinal cord, which conveycommands from the brain to the rest of the body, and convey sensoryinformation from the nerves below the neck to the brain. Even minorspinal injuries can be debilitating to the patient, and major spinalinjuries can be catastrophic. The loss of the ability to bear weight orpermit flexibility can immobilize the patient. Even in less severecases, small irregularities in the spine can put pressure on the nervesconnected to the spinal cord, causing devastating pain and loss ofcoordination.

The spinal column is a bio-mechanical structure composed primarily ofligaments, muscles, bones, and connective tissue that forms a series ofvertebral bodies stacked one atop the other and intervertebral discsbetween each vertebral body. The spinal column provides support to thebody and provides for the transfer of the weight and the bendingmovements of the head, trunk and arms to the pelvis and legs; complexphysiological motion between these parts; and protection of the spinalcord and the nerve roots.

The stabilization of the vertebra and the treatment for spinalconditions may be aided by a surgically implanted fixation device whichholds the vertebral bodies in proper alignment and reduces the patient'spain and prevents neurologic loss of function. Spinal fixation is afrequently used medical procedure. Spinal fixation systems can besurgically implanted into a patient to aid in the stabilization of adamaged spine or to aid in the correction of other spinal deformities.Existing systems can use a combination of rods, plates, pedicle screws,bone hooks, locking screw assemblies, and connectors, for fixing thesystem to the affected vertebrae. The system components may be rigidlylocked together to fix the connected vertebrae relative to each other,stabilizing the spine until the bones can fuse together.

Whatever the treatment, the goal remains to improve the quality of lifefor the patient. In some instances, patients who receive implants totreat the primary pathology develop a secondary condition calledjunctional disease. The junctional disease can occur at the proximal orcephalad area of spinal instrumentation and is then termed “adjacentsegment pathology.” Clinical Adjacent Segment Pathology (CASP) refers toclinical symptoms and signs related to adjacent segment pathology.Radiographic Adjacent Segment Pathology (RASP) refers to radiographicchanges that occur at the adjacent segment. A subcategory of CASP andRASP that occurs at the proximal end of the instrumentation is termedproximal junctional kyphosis (PJK). PJK may be defined in severalmanners and commonly is specified as kyphosis measured from one segmentcephalad to the upper end instrumented vertebra to the proximalinstrumented vertebra with abnormal value defined as 10° or greater. Inpractice, this can often mean that the patient's head and/or shouldersmay tend to fall forward to a greater degree than should normally occur.Sometimes the degree is significant.

Adjacent segment pathology can occur as either a degenerative, traumaticor catastrophic condition and sometimes as a result from a combinationof factors. Degenerative conditions are ones that occur over a period oftime, normally 5 or 6 years but can occur at an accelerated rateparticularly with altered mechanics related to spinal fusion. As aresult, the patient's head and/or shoulder region(s) may fall forwardgradually over time. Traumatic and catastrophic conditions may occur asa generally sudden shifting of the vertebral body immediately cephaladto the upper end instrumented vertebra and can lead to sudden changes inspinal alignment with marked symptoms noted by the patient.

Whether the condition is degenerative, traumatic, or catastrophic, theexact cause of adjacent segment pathology can be uncertain. Adjacentsegment pathology and more specifically PJK can be a result of excessstrain and stress on the proximal instrumented spinal segment which isthen at least partially transferred to the bone structures, disc,ligaments and other soft tissues, causing a loss of normal structuralintegrity and mechanical properties. The resultant effect can be aforward (i.e. kyphotic) shift of the adjacent non-instrumented vertebralbody. One such theory is that this strain and stress can be caused bysuboptimal alignment and/or balance of the screw and rod construct.Another theory is that the rigidity of the screw and rod constructcauses the problem in that the transition from a motion-restrainedsegment to a motion-unrestrained segment is too much for thenon-instrumented (unrestrained) segment to handle over time. Yet anothertheory speculates that the specific level at which the proximalinstrumented vertebra is located is of vital importance in that somelevels may be better suited to handle a proximal termination of afixation construct than others.

Thus there remains an urgent need for improvements and new systems forspinal fixation with at least a specific goal of preventing theoccurrence of or reducing the degree of adjacent segment pathology andfailures occurring at the distal junction (DJK), proximal junction(PJK), or both. The devices, implants, methods and techniques describedherein are directed towards overcoming these challenges and othersassociated with spinal fixation.

SUMMARY OF THE INVENTION

The problems noted above, as well as potentially others, can beaddressed in this disclosure by a system for spinal fixation with anon-rigid portion, e.g., a soft zone, at one or more of the caudal orcephalad terminus. Various devices and techniques are described hereinfor transition from a rigid fixation device to a less rigid, moreflexible support structure applied to a “soft zone” that may help sharethe stress created on the spinal levels caused by the fixed levels belowand/or above. In some embodiments, the soft zone is provided byterminating the spinal fixation system with one or more of a flexibletether assembly, a telescoping rod, or a dampening rod.

In a first aspect, a system for spinal fixation is provided comprising:a first bone anchor, anchored to a first vertebra in a subject, thefirst bone anchor comprising a first bone fastener attached to a firstrod housing; a rigid spinal rod seated in the first rod housing torestrict translation of the rigid spinal rod relative to the first boneanchor; a second bone anchor, anchored to a second vertebra in thesubject, the second bone anchor comprising a second bone fastenerattached to a second rod housing, wherein the rigid spinal rod is seatedin the second rod housing to restrict translation of the rigid spinalrod relative to the second bone anchor; and a compressible spinalconnector, connected to the first or second bone anchor, and anchored toa third vertebra in the subject, the compressible spinal connectorcomprising a modulation mechanism for modulating at least one of thetension on the compressible spinal connector or the resistance tocompression of the compressible spinal connector, wherein saidmodulation occurs in response to a remote signal.

In a second aspect, a spinal tether assembly for providing non-rigidintervertebral support is provided, comprising: a flexible tether; andan adjustable tensioner connected to exert tension on the flexibletether, the adjustable tensioner comprising a first magnet mounted torotate in response to a spinning magnetic field; and a tensioningmechanism configured to convert rotation of the magnet to a decrease orincrease of tension on the flexible tether, depending on the directionof the first magnet's rotation.

In a third aspect, a dampening spinal rod to adjust friction againsttension and compression is provided, comprising: an elongate rigidportion for insertion into a bone anchor; a flared portion for receivinga terminal end of a second spinal rod, the flared portion comprising arod cavity of sufficient diameter to accept the second spinal rod, and afriction control mechanism configured to modulate friction between thesecond spinal rod and said dampening spinal rod in response to a remotesignal.

In a fourth aspect, a method of fixing the relative positions of a firstvertebra and a second vertebra in a subject is provided, the methodcomprising: anchoring a first bone anchor to the first vertebra, thefirst bone anchor comprising a first bone fastener attached to a firstrod housing; seating a rigid spinal rod in the first rod housing torestrict translation of the rigid spinal rod relative to the first boneanchor; anchoring a second bone anchor to the second vertebra, thesecond bone anchor comprising a second bone fastener attached to asecond rod housing, seating the rigid spinal rod in the second rodhousing to restrict translation of the rigid spinal rod relative to thesecond bone anchor; connecting a compressible spinal connector to thesecond bone anchor, the compressible spinal connector comprising amodulation mechanism for modulating at least one of the tension on thecompressible spinal connector or the resistance to compression of thecompressible spinal connector, wherein said modulation occurs inresponse to a remote signal; anchoring the compressible spinal connectorto a third vertebra in the subject; and transmitting the remote signalto the modulation mechanism post-operatively, to cause said modulationto occur.

In another aspect, disclosed herein is a system for spinal fixation, thesystem comprising: a first bone anchor anchored to a first vertebra of asubject, the first bone anchor comprising a first bone fastener attachedto a first rod housing; a second bone anchor anchored to a secondvertebra in the subject, the second bone anchor comprising a second bonefastener attached to a second rod housing; a rigid spinal rod seated inthe first rod housing to restrict translation of the rigid spinal rodrelative to the first bone anchor and in the second rod housing torestrict translation of the rigid spinal rod relative to the second boneanchor; and a compressible spinal connector, connected to the rigidspinal rod and anchored to a third vertebra in the subject, thecompressible spinal connector modulates, in response to a signalexternal to the system, at least one of: tension on the compressiblespinal connector; or resistance to compression of the compressiblespinal connector. In some cases, the signal is a magnetic field. In somecases, at least one of the first bone anchor, the second bone anchor,the rigid spinal rod, and the compressible spinal connector comprises anon-absorbable biocompatible material that is non-ferromagnetic. In somecases, the compressible spinal connector comprises a tether assembly. Insome cases, the tether assembly comprises a first flexible tether atleast partially wrapped around a structure of the third vertebra andconnected to the rigid spinal rod to exert tension between the thirdvertebra and the rigid spinal rod. In some cases, the compressiblespinal connector comprises an adjustable tensioner configured to varythe tension on the first flexible tether. In some cases, the firstflexible tether is constructed of a non-absorbable biocompatiblematerial. In some cases, the tether assembly comprises a second flexibletether encircling the structure or a second structure of the thirdvertebra and a third structure of a fourth vertebra, and wherein thecompressible spinal connector comprises an adjustable tensionerconfigured to vary the tension on the second flexible tether. In somecases, the adjustable tensioner comprises a turnbuckle comprising athreaded first end coupler, a second end coupler, and a rotatable magnetthat rotates in response to a magnetic field, and wherein the rotatablemagnet is connected to the threaded first end coupler to cause thethreaded first end coupler to rotate about its longitudinal axis whenthe rotatable magnet rotates. In some cases, the adjustable tensionercomprises a spool about which the flexible tether is wound, and whereinrotation of a spool magnet drives rotation of the spool. In some cases,the adjustable tensioner comprises a locking mechanism configured tomaintain tension on the flexible tether when engaged. In some cases, thecompressible spinal connector comprises a dampening spinal rod that iscompressible and expandable. In some cases, the dampening spinal rodcomprises: an elongate rigid portion for insertion into a bone anchor; aflared portion for receiving a terminal end of a second elongate rigidportion, the flared portion comprising a rod cavity of diametersufficient to accept the second elongate rigid portion. In some cases,the compressible spinal connector comprises a friction brake configuredto vary the resistance of the dampening rod to the compression or thetension. In some cases, the friction brake comprises a set screw in athreaded channel positioned to exert compressive force on a spring, saidspring positioned to exert compressive force against both thecompression and tension of the dampening rod. In some cases, the setscrew is magnetic and rotates in the threaded, channel in response to amagnetic field. In some cases, the compressible spinal connectorcomprises a telescoping spinal rod positioned within the second rodhousing, wherein the telescoping spinal rod comprises: a rod magnetconfigured to rotate when exposed to a magnetic field and cause thetelescoping spinal rod to either extend or collapse depending on adirection of the magnetic field; a first elongate element containing acavity; and a second elongate element dimensioned to at least partiallyfit within the cavity, and having an internally threaded region, whereinthe compressible spinal connector comprises a lead screw coupled torotate when the rod magnet rotates, and comprising an externallythreaded region engaged to the internally threaded region of the secondelongate element, such that rotation of the lead screw causes the secondelongate element to translate relative to the first elongate element. Insome cases, a second rigid spinal rod is seated in an additional rodhousing of an additional bone anchor that is anchored in at least one ofthe first and second vertebrae. In some cases, a transverse connector isfastened to the first rigid spinal rod and the second rigid spinal rod.In some cases, the vertebral fixation system herein further comprises: athird bone anchor comprising a third bone fastener and a third rodhousing, anchored to the first vertebra; a fourth bone anchor,comprising a fourth bone fastener and a fourth rod housing, anchored tothe second vertebra; a second rigid spinal rod seated in the third rodhousing and the fourth rod housing, wherein the compressible spinalconnector comprises: a first flexible tether at least partially wrappedaround a structure of the third vertebra and connected to the first andsecond rigid spinal rods to exert tension between the third vertebra andthe first and second rigid spinal rods, and a second flexible tetherencircling the structure of the third vertebra and a second process of afourth vertebra, wherein the compressible spinal connector comprises anadjustable tensioner connected to the first and second rigid spinal rodsand the first or second flexible tethers, the adjustable tensionercomprising a magnet mounted to rotate in response to a magnetic fieldthereby increasing or decreasing tension on the first or second flexibletether depending on a direction of rotation of the magnet.

In another aspect, disclosed herein a system for spinal fixationcomprising: a first bone anchor anchored to a first vertebra of asubject, the first bone anchor comprising a first bone fastener attachedto a first rod housing; a second bone anchor anchored to a secondvertebra in the subject, the second bone anchor comprising a second bonefastener attached to a second rod housing; a third bone anchorcomprising a third bone fastener and a third rod housing, anchored tothe first vertebra; a fourth bone anchor, comprising a fourth bonefastener and a fourth rod housing, anchored to the second vertebra; arigid spinal rod seated in the first rod housing to restrict translationof the rigid spinal rod relative to the first bone anchor and in thesecond rod housing to restrict translation of the rigid spinal rodrelative to the second bone anchor; a second rigid spinal rod seated inthe third rod housing and the fourth rod housing, a compressible spinalconnector, connected to the rigid spinal rod and anchored to a thirdvertebra in the subject, the compressible spinal connector modulates, inresponse to a signal external to the system, at least one of: tension onthe compressible spinal connector; or resistance to compression of thecompressible spinal connector, wherein the compressible spinal connectorcomprises: a first flexible tether at least partially wrapped around astructure of the third vertebra and connected to the first and secondrigid spinal rods to exert tension between the third vertebra and thefirst and second rigid spinal rods, and a second flexible tetherencircling the structure of the third vertebra and a second process of afourth vertebra, and wherein the compressible spinal connector comprisesa magnet mounted to rotate in response to a magnetic field therebyincreasing or decreasing tension on the first or second flexible tetherdepending on a direction of rotation of the magnet.

In yet another aspect, disclosed herein is a system for spinal fixationcomprising: a compressible spinal connector connected to a rigid spinalfixation rod implanted in a subject and anchored to a vertebra that ismovable relative to the rigid spinal fixation rod, wherein thecompressible spinal connector comprises: a first flexible tether atleast partially wrapped around a structure of the vertebra and connectedto the rigid spinal rod to exert tension between the vertebra and therigid spinal rod; and a second flexible tether encircling the structureor a second structure of the vertebra and a third of a second vertebra;and a magnet mounted to rotate in response to an external magnetic fieldthereby increasing or decreasing tension on the first or second flexibletether depending on a direction of rotation of the magnet.

Additional aspects and advantages of the present disclosure will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein only illustrative embodiments of thepresent disclosure are shown and described. As will be realized, thepresent disclosure is capable of other and different embodiments, andits several details are capable of modifications in various obviousrespects, all without departing from the disclosure. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings (also “Figure” and “FIG.” herein), of which:

FIG. 1 shows an example of a vertebral fixation system disclosed herein,in accordance with embodiment(s) disclosed herein;

FIG. 2 shows a side view of an exemplary embodiment of the dampeningspinal rod as part of a vertebral fixation system, in accordance withembodiment(s) disclosed herein;

FIG. 3 shows a dorsal (posterior) view of an exemplary embodiment of thevertebral fixation system;

FIG. 4 shows a perspective view of an exemplary embodiment of atensioner that modules the tension on a flexible tether of a vertebralfixation system disclosed herein;

FIG. 5 shows an exemplary embodiment of a locking mechanism for lockinga tensioner of the vertebral fixation system disclosed herein;

FIG. 6 shows a cross-sectional view of an exemplary embodiment of atelescoping spinal rod of the vertebral fixation system disclosedherein;

FIG. 7 shows an exemplary embodiment of a turnbuckle tensioner formodulating the tension on a flexible tether of the vertebral fixationsystem disclosed herein;

FIG. 8 shows a dorsal (posterior) view of an exemplary embodiment of thespinal fixation systems; and

FIG. 9 shows a top view of an exemplary embodiment of the externalcontrol device of the vertebral fixation system disclosed herein.

DETAILED DESCRIPTION

Illustrative embodiments of a system for spinal fixation, parts, andmethods for use thereof, are described below. In the interest ofclarity, not all features of an actual implementation are described inthis specification. It will of course be appreciated that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The system for spinal fixation, parts, and methods foruse thereof disclosed herein boasts a variety of inventive features andcomponents that warrant patent protection, both individually and incombination.

While various embodiments of the invention have been shown and describedherein, it will be obvious to those skilled in the art that suchembodiments are provided by way of example only. Numerous variations,changes, and substitutions may occur to those skilled in the art withoutdeparting from the invention. It should be understood that variousalternatives to the embodiments of the invention described herein may beemployed.

Unless otherwise defined, all technical terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art towhich this invention belongs. As used in this specification and theappended claims, the singular forms “a,” “an,” and “the” include pluralreferences unless the context clearly dictates otherwise. Any referenceto “or” herein is intended to encompass “and/or” unless otherwisestated. As used in this specification and the appended claims, “about,”or “approximately” indicate ±0.1 to ±15% difference from the subjectmatter that is being characterized. As a nonlimiting example, about 90degrees may indicate 89.5 degrees, 88 degrees, 100 degrees, or anynumber(s) in the range from 75 degrees to 105 degrees.

Illustrative embodiments of a system for spinal fixation, parts, andmethods for use thereof, are described below. In the interest ofclarity, not all features of an actual implementation are described inthis specification. It will of course be appreciated that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The system for spinal fixation, parts, and methods foruse thereof disclosed herein boasts a variety of inventive features andcomponents that warrant patent protection, both individually and incombination.

As disclosed herein, “caudal” is equivalent to “distal” and “cephalad”is equivalent to “proximal.”

The present disclosure includes a variety of transitional or terminalcomponents that may be implanted or otherwise used as part of avertebral fixation system disclosed herein, equivalently as a spinalfixation system. In some embodiments, the vertebral fixation systemsdisclosed herein is to decrease the potential for subsequent developmentof junctional disease or failure, for example, subsequent to a spinalsurgical procedure. In the examples shown, only the cephalad most level(for terminal hardware) or levels (for multilevel transitional hardware)of the fixation system (e.g. those utilizing the exemplary componentsdescribed herein) are illustrated. It should be appreciated, however,that the entire vertebral fixation system may extend any number oflevels from a single level construct to a long construct spanningmultiple spinal levels and multiple spinal regions from the lumbosacralto cervical regions (such as the exemplary embodiment illustrated inFIG. 1), and/or with any variety of combinations of known anchors, rods,and connectors. It should also be appreciated that the exemplaryterminal and/or transitional components may additionally oralternatively be utilized at the caudal or distal end of the fixationconstruct. In some embodiments, the vertebral fixation systems describedherein is used along any aspect of the spine (e.g. anterior, posterior,antero-lateral, postero-lateral). In some embodiments, the vertebralfixation systems are particularly suited for implantation along aposterior aspect of the spine.

In some embodiments, the vertebral fixation system disclosed hereincomprises a first bone anchor, anchored to a first vertebra in asubject, the first bone anchor comprising a first bone fastener attachedto a first rod housing. A rigid spinal rod can be seated in the firstrod housing to restrict translation of the rigid spinal rod relative tothe first bone anchor. The rigid spinal rod can also be seated in therod housing of a second bone anchor, anchored to a second vertebra inthe subject, so as to restrict translation of the rigid spinal rodrelative to the second bone anchor. A compressible spinal connector canbe connected to the first and/or second bone anchor and anchored to athird vertebra in the subject. The compressible spinal connector, mayhave a modulation mechanism for modulating either the tension on thecompressible spinal connector or its resistance to compression (orboth), thereby allowing adjustment between the second bone anchor andthe third vertebrae. The modulation can occur in response to a signalexternal to the vertebral fixation system. Consequently, modulation ofthe tension and/or resistance to compression may not require access tothe vertebral fixation system through the patient's tissues, and may beperformed post-operatively. The external, remote signal may be, forexample, an electromagnetic signal. A specific example of the remote,external signal is a spinning magnetic field.

FIG. 1 illustrates an exemplary embodiment of the vertebral fixationsystems. In this particular embodiment, the illustrated vertebralfixation system 5 is a screw-and-rod construct adapted for implantationalong the posterior aspect of the human spinal column. In thisparticular embodiment, the vertebral fixation system 5 includes a pairof elongate rods 25 a, 25 b (one rod shown) dimensioned to span multiplevertebral levels and a plurality of bone anchors, e.g., 10, 30. The boneanchors can be threaded bone anchors and/or hook-type bone anchors. Thevertebral fixation system can also include a plurality of transverseconnectors or cross connectors dimensioned to rigidly engage each of theelongate rods 25 a, 25 b so as to hold each rod in place relative to theother. The transverse connectors or cross connectors may be provided asfixed connectors or adjustable connectors, in any quantity that isrequired by the surgeon performing the implantation surgery.

It is contemplated that any of the examples of bone anchors and othertransition structures or elements of the vertebral fixation systemsdescribed herein may be substituted for the cephalad bone anchors and/orcaudal bone anchors which are traditionally rigid and identical to theother bone anchors used throughout the system. It is also contemplatedthat the examples of flexible or compressible spinal connectors,adjustable connectors, any other structure/elements described herein mayreplace existing hardware at the cephalad and/or caudal terminus of thetraditional vertebral fixation system such that there is no additionalsurgical footprint realized. It is further contemplated that theexamples of the vertebral fixation system and its structure elementsdescribed herein may augment existing hardware at the cephalad and/orcaudal terminus of the traditional vertebral fixation system such thatthere is additional added surgical footprint realized. This may beapplicable with the various embodiments that can be installed withminimal disruption of additional muscle tissue and/or ligamentstructure. Junctional disease or failure can be a problem at either thecephalad or caudal terminus (or both) of vertebral fixation systems.Therefore, although the various examples disclosed herein may bedescribed in terms of cephalad terminus and proximal joint disease (forease of disclosure) it is to be understood that any of the exampleembodiments are also applicable and may be used at the caudal terminusand distal joint disease of the vertebral fixation system withoutdeviating from the scope of this disclosure.

According to one example, a spinal fixation system 5, like that shown inFIG. 1, is applied to the spinal levels to be fixed. Above the fixedlevels or the fixed zone, a soft-zone can be created by applyingnon-rigid support elements such as tethers or adjustable rods that limitcertain motion and reduce stress, to the levels of the soft-zone andabove, while not inhibiting all motion. The tension applied to thesupport elements in the soft zone can be adjusted post-operatively andnon-invasively to account for changing dynamics in the body, or for anyother reason deemed desirable.

The components in the vertebral fixation system can be constructed fromone or more non-absorbable biocompatible materials. Specificnon-limiting examples of such suitable materials include titanium,alloys of titanium, steel, and stainless steel. Parts of the system canbe made from non-metallic biocompatible materials, which includealuminum oxide, calcium oxide, calcium phosphate, hydroxyapatite,zirconium oxide, and polymers such as polypropylene. Interference with amagnetic field (e.g., the external signal) can be reduced byconstructing one or more portions of the system from a nonmagnetic,non-ferromagnetic, or weakly magnetic material. Specific examples ofsuch nonmagnetic non-absorbable biocompatible material include titanium,alloys of titanium, aluminum oxide, calcium oxide, calcium phosphate,hydroxyapatite, zirconium oxide, and polymers such as polypropylene.Examples of weakly magnetic materials include paramagnetic materials anddiamagnetic materials. In a specific embodiment, the weakly magneticmaterial is austenitic stainless steel.

The first, second, and third vertebrae may be adjacent or non-adjacentto one another, in any combination. Thus it is contemplated that thefirst vertebra can be adjacent to the second, which can be adjacent tothe third; the first vertebra can be nonadjacent to the second, whichcan be adjacent to the third; the first vertebra can be nonadjacent tothe second, which can be nonadjacent to the third; and that the firstvertebra can be adjacent to the second, which can be nonadjacent to thethird.

Referring to FIG. 3, in a particular embodiment, the vertebral fixationsystem 5 disclosed herein comprises a first bone anchor 10, anchored toa first vertebra in a subject, the first bone anchor comprising a firstbone fastener (e.g., a threaded shank of the bone anchor) attached to afirst rod housing 20. A set screw 15 may be used to lock the rod in thefirst rod housing. A rigid spinal rod 25 a can be seated in the firstrod housing 20 to restrict translation of the rigid spinal rod relativeto the first bone anchor 10. The rigid spinal rod can also be seated inthe rod housing of a second bone anchor 30, anchored to a secondvertebra in the subject, so as to restrict translation of the rigidspinal rod 25 a relative to the second bone anchor 30. A compressiblespinal connector can be connected to the first or second bone anchor andanchored to a third vertebra 301 in the subject. In this particularembodiment, the vertebral fixation system 5 includes a compressiblespinal connector including one or more tether assemblies 95. In thetether assembly 95, the modulation mechanism is an adjustable tensioner350 configured to vary the tension on a flexible tether 97 a. This maycontrol the tension between the third bone anchor 300 and/or the firstbone anchor 10 and the third vertebra 301. In this exemplary embodiment,the tethers 97 a, 97 b may be attached between the fixation hardware(e.g., bone anchor(s) and elongate rod(s)) and the soft-zone 98 (e.g.one or more non-fixed levels above), and/or directly between the boneelements of one or more fixed levels 99 and the soft-zone, and/orbetween two or more of the non-fixed levels in the soft-zone. As anonlimiting example, a tether 97 b may be wrapped around spinousprocesses of two vertebrae 302, 402 in the soft zone. As anotherexample, a tether may be connected to a rigid spinal rod 25 a or 25 band at least partly wrapped around a spinous process of one vertebra inthe soft zone 302.

The tether 97 a, 97 b may be formed of any material suitable for medicaluse. For example, the tether may be made from allograft tendon,autograft tendon, braided, woven, or embroidered polyethylene, braided,woven, or embroidered polyester, polyether ether ketone (PEEK), orpolyetherketoneketone (PEKK). In some instances, the tether 97 a, 97 bmay be formed of elastic material.

FIG. 3 depicts multiple tethers 97 a, 97 b applied to the spine in thesoft zone 98 adjacent to the fixed zone 99 and connected to thevertebral fixation system 5 by adjustable tensioner(s) 350. Nonlimitingexamples of the adjustable tensioner, equivalently herein as theadjustable tension connector, include an adjustable tension tether-rodconnector, an adjustable tension cross-connector shown in FIG. 8, aturnbuckle shown in FIG. 7, and a spool shown in FIG. 4. It will beappreciated that while shown in use together, either of these connectorsmay also be used on their own. and in any configuration desired. In use,once the tethers 97 a, 97 b are connected to the fixation system 5 andthe tether(s) coupled to the desired bone structure (or other boneconnection element) the tension on the tether(s) can be adjusted. Later,the tension on the tether can be adjusted post-operatively using anexternal device, e.g., 155 in FIG. 9, that controls the adjustabletensioners 350.

The vertebral fixation system disclosed herein and structure elementsthereof may be unilateral, in which the network of bone anchors and rodsis present on one side of the spine. The vertebral fixation systemdisclosed herein and structure elements thereof may be bilateral and ispresent on either side of the spine. Such a bilateral system maycomprise a second rigid spinal rod 25 b seated in an additional rodhousing of an additional bone anchor that is anchored in at least one ofthe first and second vertebrae.

Continuing to refer to FIG. 3, in this embodiment, the system 5comprises a third bone anchor 300 (comprising a third bone fastener anda third rod housing 310) anchored to the first vertebra, and a fourthbone anchor 320 (comprising a fourth bone fastener and a fourth rodhousing 340) anchored to the second vertebra. The third bone anchor mayfurther include a set screw 305 that lock the rod in the rod housing,similarly, the fourth bone anchor may further comprise a set screw 330.A second rigid spinal rod 25 b is seated in the third rod housing 310and the fourth rod housing 340, running substantially parallel to thefirst rod 25 a. The compressible spinal connector can include anadjustable tether assembly 95 having a first flexible tether 97 a atleast partially wrapped around a spinous process of the third vertebraand connected to both of the first 25 a and second rigid spinal rods 25b to exert tension between the third vertebra and the first 25 a andsecond rigid spinal rods 25 b. The adjustable tether assembly 95 mayalso include a second flexible tether 97 b encircling the spinousprocess of the third vertebra 301 and a spinous process of a fourthvertebra 401. The compressible spinal connector can also include anadjustable tensioner 350 connected to the first 25 a and second 25 brigid spinal rods. The adjustable tensioner 350 can include a magnetmounted to rotate in response to a spinning magnetic field, and aconnection of the magnet to the tether(s) configured to increase ordecrease the tension on the second flexible tether 97 b depending on thedirection of rotation of the magnet. In some embodiments, the tether(s)may be wrapped around or otherwise attached to any structure element ofa vertebra other than the spinous process. The adjustable tensioner 350extends or retracts the tether depending on the direction of thespinning magnetic field, either reducing or increasing the tensionrespectively.

As shown in FIGS. 3 and 8, one or more transverse connectors or crossconnectors 410 adjustably or fixedly fastened to the first rigid spinalrod 25 a and the second rigid spinal rod 25 b may be present foradditional stability.

FIG. 3 shows an exemplary embodiment of the adjustable tension crossconnector. In this embodiment, the adjustable tension cross connector410 includes a pair of rod connectors 430 that couple and lock theadjustable tension cross connector to each of the rods 25 a, 25 b.Support arms 420 extends from the rod connector and supports the housingof the adjustable tensioner 350.

FIG. 8 shows another exemplary embodiment of the adjustable tensioncross connector 410. In this particular embodiment and in the embodimentshown in FIG. 3, the adjustable tensioner connector may providestability and rigidity to the elongate rods 25 a, 25 b. The adjustablecross-connector can be fixedly or adjustably anchored to the vertebrausing elements such as bone anchors, for example, rod connectors 430,pedicle screws, set screws 440, and/or hooks. The adjustablecross-connector can hold the elongate rods 25 a, 25 b in a desiredposition. The adjustable cross-connector can connect to the adjustabletensioner via support arm(s) 420 thus provide support for the adjustabletensioner 350.

Referring to FIG. 7, in a particular embodiment, the adjustabletensioner includes a turnbuckle 105 comprising a threaded first endcoupler 110, a second end coupler 115, and a rotatable magnet 120 thatrotates in response to a spinning magnetic field and that is connectedto the threaded first end coupler 110 to cause the threaded first endcoupler 110 to rotate about its longitudinal axis when the rotatablemagnet 120 rotates. FIG. 7 shows a cross-section of the turnbuckle 105including a cylindrical magnet 125 oriented to rotate around itslongitudinal axis when exposed to a spinning magnetic field in the rightorientation. The threaded first end coupler 110 can be a hook 130 with athreaded shank 135. The threaded shank 135 runs through a threadedchannel 140 in the housing 145 of the turnbuckle 105, which translatesrotation of the shank 135 into translation of the hook 130.Alternatively, the magnet 125 itself may contain one or more threadedpassages 150 that are engaged with the threaded shank(s) 135. As aresult, the hook 130 can be extended or retracted by rotating the shank135. In some embodiments of the turnbuckle, both end couplers 110, 115are hooks with threaded shanks, the threads are oriented such that whenthe rotatable magnet rotates in a given direction the two hookstranslate in opposite directions (i.e., they either converge or divergealong their shared longitudinal axis). When the hooks are caused toconverge, it can increase tension on the connected spinal structure.

In some embodiments, the turnbuckle 105 can be attached to the spinousprocess or lamina of two vertebrae (either directly, or via tetherslooped around the lamina or spinous process), and the tension betweenthe vertebrae can be adjusted post-operatively and non-invasively usingthe external adjustment device to rotate the turnbuckle magnet 120. Insome embodiments, such two vertebrae are adjacent to each other. In someembodiments, such two vertebrae are not adjacent to each other. Thoughshown only across a single level, turnbuckles 105 can be used atmultiple levels. According to one example, the turnbuckles 105 can beused selectively to set the tension differently at each level. By way ofexample, the tension can start out higher closest to the fixed spinallevels, and be sequentially decreased over a series of levels throughthe soft-zone.

In some embodiments, a pair of turnbuckles is used bilaterally andcoupled to tethers looped around the lamina and the superior andinferior coupled vertebrae. A distraction device can also be positionedbetween two spinous processes across a single level. It is contemplatedthat the distraction device can include a magnetically driven expansiondevice (such as one utilizing a lead screw coupled to a magnet, similarto that described below, to create linear expansion). This way, bothflexion and extension can be effectively controlled, and adjustedpost-operatively between two vertebrae. Taking it a step further, theaddition of a rotatable element 220 within the disc to allow twoadjacent vertebrae to rotate relative to each other, can facilitatescoliosis correction, e.g., in both the sagittal and coronal planes,using the adjustable turnbuckle 105 and distraction device(s).

One embodiment of the adjustable tensioner includes a spool about whichthe flexible tether is wound, and wherein rotation of a spool magnetdrives rotation of the spool. An example of such an embodiment is shownin FIG. 4. In this particular embodiment, the spool 165 is connected toa body 175 having a rod passage 180 that couples to the rod 25 a, 25 b.A setscrew 205 or other element may be used to lock the body 175 to therod 25 a, 25 b. The spool 165 is rotatably connected to the body 175.The tether 97 a, 97 b is attached to the spool 165 such that when thespool 165 rotates the tether 97 a, 97 b is wound up on the spool 165 tocreate tension. A first end 185 of the tether may be attached to thespool 165 with the second end 190 being otherwise attached to the targetbone, another adjustable connector, or to itself (e.g. creating a loopthat can be attached to the target bone, or both ends of the tether(185, 190) may be coupled to the spool 165 creating a loop to attach toor around the bone such that both ends of the tether (185, 190) arespooled up together). The spool magnet 170 can be driven by applicationof a magnetic field to rotate the spool 165. The spool magnet 170 may bea single cylindrical magnet poled north and south across its diameter toform two 180 degree sectors, as in FIG. 4. Alternatively, a quadrupoleor multipole magnet may be used. The spool 165 may include a lockingmechanism, such as a spool 165 and a ratchet mechanism, to maintain thetension applied. The locking mechanism may be externally controlledsimilar to the spool magnet 170 such that it can be locked and unlockedif adjustment is needed.

According to one embodiment, the locking mechanism may be a set screw,e.g., 205 in FIG. 2, situated to inhibit rotation of the spool 165, whenengaged. The set screw may be a magnetically driven set screw, orientedsuch that the external drive controller 155 can be positioned to driveonly one of the set screw and drive magnet 230, and then prepositionedto drive the other. Alternatively, a locking pin or shaft could beadvanced with the set screw to inhibit rotation of the spool 165.

In some embodiments, an external device 155, e.g., magnet 230 a, 230 bas in FIG. 9, may drive a gear assembly 380, as shown in FIG. 5, thatrotates two opposing ratchet wheels 385 through which the tether 97 a,97 b is passed, so that the tether may be locked when the gear assemblydoes rotate and unlocked when the gear assembly rotates.

In some embodiments, the compressible spinal connector herein includes adampening rod. The dampening rod can be a rod that is both expandableand compressible, and the resistance to expansion and compression iscontrolled by means of the modulation mechanism herein. In someembodiments, the modulation mechanism includes a friction brake. Thedampening rod can accommodate dynamic travel or length adjustment of therod between the fixed connectors. The friction brake can include a setscrew that is itself magnetic, or non-magnetic itself but connected to amagnet (“brake magnet”) that may be controllable via an externaladjustment device. The degree of tension and support provided by thedampening rod can be controlled by increasing or decreasing frictionwith the set screw. Some embodiments of the friction brake can also lockdown the rod entirely, to prevent any expansion or compression, shouldit later become necessary to fix one or more levels in the soft-zone. Anembodiment of the dampening rod 235 is shown in FIG. 2. In thisembodiment, each damping rod 235 has a wider bell region 395 and anarrower tail region 400. The tail 400 is about the diameter of anordinary spinal rod. The bell 395 is open on the inside, and isdimensioned to accommodate a spinal rod (or the tail of anotherdampening rod 235). As shown in FIG. 2, the friction brake may be, forexample, a set screw 205 in a threaded channel positioned to exertcompressive force on a spring 250. The spring 250 can be positioned toexert compressive force against both the compression and expansion ofthe dampening rod 235. In this particular embodiment shown in FIG. 2,the spring 250 is positioned to exert compressive force on the tailportion 400 of an adjacent dampening rod 235. The spring 250 can be awave spring, although other kinds of springs (e.g., helical) arecontemplated as well. The set screw may be magnetic, or coupled to amagnet, such that a rotating magnetic field in the proper orientationmay cause the set screw to rotate, either increasing or decreasing thecompressive force that exerts the friction.

In some embodiments, the vertebral fixation system herein may be usedtogether with a telescoping rod. As a nonlimiting example, thetelescoping rod may be implanted at levels above a vertebral fixationsystem, e.g., in patients that are at high risk of developing PJK orother adjacent segment diseases. The telescoping rod may be implanted asa prophylactic and used if needed to extend the length for pain relief.An example of the telescoping rod is shown in FIG. 6. In this particularembodiment, the telescoping spinal rod 255 comprises a rod magnet 260configured to rotate when exposed to a spinning magnetic field and causethe telescoping spinal rod 255 to either extend or collapse depending onthe direction of the spinning magnetic field. The rod magnet 260 may bea cylindrical permanent magnet (such as a ferrimagnet), but may beanother type of magnet. The telescoping rod may include a first elongateelement 265 containing a cavity 270, into which fits a second elongateelement 275. There may be a dynamic seal between the first 265 andsecond elongated elements 275, to ensure that no bodily fluids enter thetelescoping rod. A thrust bearing may be included to reduce frictionbetween the spinning magnetic element and the housing. The secondelongate element 275 can have an internally threaded region 280 that isengaged to a lead screw 285 coupled to rotate when the rod magnet 260rotates, and comprising an externally threaded region 290, such thatrotation of the lead screw 285 causes the second elongate element 275 totranslate along the longitudinal direction relative to the firstelongate element 265. Thus, when the magnet 260 rotates, the lead screw285 can also rotate. The threaded interface between the lead screw 285and the second elongated element 275 can then cause the second elongatedelement 275 to translate, with the translational direction beingdependent upon the rotational direction of the magnet 260. The threadson the internally threaded region 280 of the first elongate element 265may be integral, or they may be on the inner surface of anotherstructure, such as a threaded nut.

Whenever the adjustable tensioner is actuated by the rotation of amagnet 120, as a safety precaution, a magnetic immobilization plate 295may be positioned sufficiently close to the rotatable magnet 120 tocause the rotatable magnet 120 to adhere to the immobilization plate 295in the absence of a strong external magnetic field. The magneticimmobilization plate 295 can hold the rotating magnet 120 in position,preventing it from rotating, until a magnetic field with a strengthabove a certain threshold is applied. Like the rotating magnet 120, theimmobilization plate may be constructed from a suitable magneticmaterial, such as a ferromagnetic material. The immobilization plate maybe used on its own, or in combination with a locking mechanism 195 asdescribed above.

Methods of using the vertebral fixation system 5 to fix the relativepositions of a first vertebra and a second vertebra in a subject areprovided herein. In some embodiments, the method comprises anchoring afirst bone anchor 10 to the first vertebra, the first bone anchor 10comprising a first bone fastener 15 attached to a first rod housing 20;seating a rigid spinal rod 25 a in the first rod housing 20 to restricttranslation of the rigid spinal rod 25 a relative to the first boneanchor 10; anchoring a second bone anchor 30 to the second vertebra, thesecond bone anchor 30 comprising a second bone fastener 33 attached to asecond rod housing 35; seating the first rigid spinal rod 25 a in thesecond rod housing 35 to restrict translation of the rigid spinal rod 25a relative to the second bone anchor 30; connecting a compressiblespinal connector to the second bone anchor 30, the compressible spinalconnector comprising a modulation mechanism for modulating at least oneof the tension on the compressible spinal connector or the resistance tocompression of the compressible spinal connector, wherein saidmodulation occurs in response to a remote signal; anchoring thecompressible spinal connector to a third vertebra in the subject; andtransmitting the remote signal to the modulation mechanismpost-operatively, to cause said modulation to occur. The vertebralfixation system may have any of the components and arrangementsdescribed above. The compressible spinal connector can be any describedas suitable for the system above, including any of the describedembodiments of the tether assembly, dampening rod, and telescoping rod.An example of an external adjustment device 155 that can be used tonon-invasively drive the modulation mechanisms described herein is shownin FIG. 9. The external adjustment device 155 is configured forplacement on or adjacent to the skin of the subject and appropriatelyaligned with the magnet to be activated, and includes at least one drivemagnet 230 a,230 b configured for rotation. The external adjustmentdevice 155 may further include a motor configured to rotate themagnet(s), whereby rotation of the drive magnet(s) of the externaladjustment device 155 effectuates rotational movement of one or moremagnets (e.g., rotatable magnet in the turnbuckle, magnetic set screw,etc.). As shown in FIG. 9, the external adjustment device 155 may havetwo magnets (230 a, 230 b). The two magnets (230 a, 230 b) may beconfigured to rotate at the same angular velocity. They may also beconfigured to each have at least one north pole and at least one southpole (i.e., a dipole magnet), and the external adjustment device 155 isconfigured to rotate one drive magnet 230 a and the other drive magnet230 b such that the angular location of the at least one north pole ofthe first drive magnet 230 a is substantially equal to the angularlocation of the at least one south pole of the second drive magnet 230 bthrough a full rotation of the first 230 a and second 230 b drivemagnets. More complex systems involving quadrupole and multipole drivemagnets are also contemplated, as is the use of one or moreelectromagnets.

The foregoing description illustrates and describes the processes,machines, manufactures, compositions of matter, and other teachings ofthe present disclosure. Additionally, the disclosure shows and describesonly certain embodiments of the processes, machines, manufactures,compositions of matter, and other teachings disclosed, but, as mentionedabove, it is to be understood that the teachings of the presentdisclosure are capable of use in various other combinations,modifications, and environments and is capable of changes ormodifications within the scope of the teachings as expressed herein,commensurate with the skill and/or knowledge of a person having ordinaryskill in the relevant art. The embodiments described hereinabove arefurther intended to explain certain best modes known of practicing theprocesses, machines, manufactures, compositions of matter, and otherteachings of the present disclosure and to enable others skilled in theart to utilize the teachings of the present disclosure in such, orother, embodiments and with the various modifications required by theparticular applications or uses. Accordingly, the processes, machines,manufactures, compositions of matter, and other teachings of the presentdisclosure are not intended to limit the exact embodiments and examplesdisclosed herein. Any section headings herein are provided only forconsistency with the suggestions of 37 C.F.R. § 1.77 and related laws orotherwise to provide organizational queues. These headings shall notlimit or characterize the invention(s) set forth herein.

While preferred embodiments of the present invention have been describedherein, it will be obvious to those skilled in the art that suchembodiments are provided by way of example only. It is not intended thatthe invention be limited by the specific examples provided within thespecification. While the invention has been described with reference tothe aforementioned specification, the descriptions and illustrations ofthe embodiments herein are not meant to be construed in a limitingsense. Numerous variations, changes, and substitutions will be apparentto those skilled in the art without departing from the invention.Furthermore, it shall be understood that all aspects of the inventionare not limited to the specific depictions, configurations, or relativeproportions set forth herein which depend upon a variety of conditionsand variables. It should be understood that various alternatives to theembodiments of the invention described herein may be employed inpracticing the invention. It is therefore contemplated that theinvention shall also cover any such alternatives, modifications,variations, or equivalents. It is intended that the following claimsdefine the scope of the invention and that methods and structures withinthe scope of these claims and their equivalents be covered thereby.

What is claimed is:
 1. A system for spinal fixation, the systemcomprising: a first bone anchor configured to be anchored to a firstvertebra of a subject, the first bone anchor comprising a first bonefastener attached to a first rod housing; a second bone anchorconfigured to be anchored to a second vertebra in the subject, thesecond bone anchor comprising a second bone fastener attached to asecond rod housing; a first spinal rod seated in the first rod housingto restrict translation of the first spinal rod relative to the firstbone anchor and in the second rod housing to restrict translation of thefirst spinal rod relative to the second bone anchor; and a dampening rodthat is compressible, expandable, and configured to be connected to thefirst spinal rod and anchored to a third vertebra in the subject,wherein the dampening rod is configured to modulate, in response to asignal external to the system, tension on the dampening rod; orresistance to the compression of the dampening rod, the dampening rodcomprising: an elongate rigid portion for insertion into a bone anchor;and a flared portion for receiving a terminal end of a second elongaterigid portion, wherein the flared portion comprises a rod cavity ofdiameter sufficient to accept the second elongate rigid portion, andwherein the system further comprises a friction brake configured to varythe resistance of the dampening rod to the compression or the tension,wherein the friction brake comprises a set screw in a threaded channelpositioned to exert compressive force on a spring, said springpositioned to exert compressive force against both the compression andtension of the dampening rod.
 2. The system of claim 1, wherein thesignal is a magnetic field.
 3. The system of claim 2, wherein at leastone of the first bone anchor, the second bone anchor, the first spinalrod, and the dampening rod comprises a non-absorbable biocompatiblematerial that is non-ferromagnetic.
 4. The system of claim 1, furthercomprising a tether assembly.
 5. The system of claim 4, wherein thetether assembly comprises a first flexible tether configured to be atleast partially wrapped around a structure of the third vertebra andconnected to the first spinal rod to exert tension between the thirdvertebra and the first spinal rod.
 6. The system of claim 5, furthercomprising an adjustable tensioner configured to vary tension on thefirst flexible tether.
 7. The system of claim 6, wherein the adjustabletensioner comprises a turnbuckle comprising a threaded first endcoupler, a second end coupler, and a rotatable magnet that rotates inresponse to a magnetic field, and wherein the rotatable magnet isconnected to the threaded first end coupler to cause the threaded firstend coupler to rotate about its longitudinal axis when the rotatablemagnet rotates.
 8. The system of claim 6, wherein the adjustabletensioner comprises a spool about which the first flexible tether iswound, and wherein rotation of a spool magnet drives rotation of thespool.
 9. The system of claim 6, wherein the adjustable tensionercomprises a lock configured to maintain tension on the first flexibletether when engaged.
 10. The system of claim 5, wherein the firstflexible tether is constructed to a non-absorbable biocompatiblematerial.
 11. The system of claim 5, wherein the tether assemblycomprises a second flexible tether configured to encircle the structureor a second structure of the third vertebra and a third structure of afourth vertebra, and wherein the system further comprises an adjustabletensioner configured to vary tension on the second flexible tether. 12.The system of claim 1, wherein the signal is a magnetic field; whereinthe set screw is magnetic; and wherein the set screw is configured torotate in the threaded channel in response to the magnetic field. 13.The system of claim 1, wherein a second spinal rod is seated in anadditional rod housing of an additional bone anchor that is configuredto be anchored in at least one of the first and second vertebrae; andwherein a transverse connector is configured to be fastened to the firstspinal rod and the second spinal rod.
 14. The system of claim 1, furthercomprising a telescoping spinal rod coupled to the first spinal rod,wherein the telescoping spinal rod comprises: a rod magnet configured torotate when exposed to a magnetic field and cause the telescoping spinalrod to either extend or collapse depending on a direction of themagnetic field; a first elongate element containing a cavity; and asecond elongate element dimensioned to at least partially fit within thecavity, and having an internally threaded region, a lead screw coupledto rotate when the rod magnet rotates; and an externally threaded regionengaged to the internally threaded region of the second elongate elementsuch that rotation of the lead screw causes the second elongate elementto translate relative to the first elongate element.
 15. A method forusing a system for spinal fixation, the method comprising: making asystem for spinal fixation available for use, the spinal fixation systemcomprising: a first bone anchor configured to be anchored to a firstvertebra of a subject, the first bone anchor comprising a first bonefastener attached to a first rod housing; a second bone anchorconfigured to be anchored to a second vertebra in the subject, thesecond bone anchor comprising a second bone fastener attached to asecond rod housing; a first spinal rod seated in the first rod housingto restrict translation of the first spinal rod relative to the firstbone anchor and in the second rod housing to restrict translation of thefirst spinal rod relative to the second bone anchor; and a dampeningspinal rod that is compressible, expandable, and configured to beconnected to the first spinal rod and anchored to a third vertebra inthe subject, wherein the dampening spinal rod is configured to modulate,in response to a signal external to the system, tension on the dampeningspinal rod or resistance to compression of the dampening spinal rod, thedampening rod comprising: an elongate rigid portion for insertion into abone anchor; and a flared portion for receiving a terminal end of asecond elongate rigid portion, wherein the flared portion comprises arod cavity of diameter sufficient to accept the second elongate rigidportion, and wherein the spinal fixation system further comprises afriction brake configured to vary the resistance of the dampening rod tothe compression or the tension, wherein the friction brake comprises aset screw in a threaded channel positioned to exert compressive force ona spring, said spring positioned to exert compressive force against boththe compression and tension of the dampening rod; anchoring the firstbone anchor to a first vertebra of a subject; anchoring the second boneanchor to a second vertebra in the subject; seating the first spinal rodin the first rod housing; connecting the dampening spinal rod to thefirst spinal rod; anchoring the dampening spinal rod to a third vertebrain the subject; and using a signal, modulating the tension on thedampening spinal rod or resistance to compression of the dampeningspinal rod.